1. Field of the Invention
The present invention relates to a machine tool using a digital spindle motor for its principal-axis motor, and more particularly, to a tapping control system in which rotation of a tapping tool (tap) by means of a digital spindle motor is synchronized with axial movement of the tap by means of a servomotor, thereby ensuring ideal tapping work.
2. Description of the Related Art
In tapping work, a tap must be advanced over a predetermined distance while it makes one revolution. Thus, the rotation and axial advance of the tap must be synchronized completely.
In association with this, high-accuracy screw or gear cutting work is conventionally performed in a manner such that rotation of a tool and its feed relative to a workpiece are synchronized. For example, a principal-axis motor for tool rotation is rotated at a predetermined command speed. Rotation of a tool-feed motor is controlled in accordance with a pulse train, generated from a pulse coder attached to the tool-rotation motor, so that the two motors are rotated in synchronism with each other. In the synchronous control of this type, speed commands are given to the individual motors, that is, a speed control loop is used as a control loop. The righting moment of the speed control loop against variations of speed is small, and the rotational speed of the tool-rotation motor is likely to change as the load varies. As the rotational speed of the tool-rotation motor varies in this manner, the two motors becomes non-synchronous thereby lowering working accuracy. When tapping a deep hole by means of a small-diameter tap, the tap may sometimes be broken.
In connection with the synchronous control of the aforementioned type, a multi-function machine tool is conventionally used for various machine works. Various tools, such as a tapping tool, cutter, etc., can be replaceably mounted on a tool holder of the machine tool. However, this machine tool has problems with working accuracy on account of the aforesaid reasons.
In a system disclosed in Japanese Patent Disclosure No. 60-155319, for example, a feed driver is used to relatively move a workpiece and a tool. Rotation of a rotation driver, which is used to relatively rotate the workpiece and the tool in accordance with the amount of feed by the feed driver, is controlled. This system, however, is intended for screw working only. Although it can therefore perform high-accuracy screw working, the system, disclosed in 60-155319 which uses a servomotor incapable of high-speed rotation, cannot perform work other than screw working despite an improvement for the attachment of various tools. In particular, the system cannot actually perform deep cutting in which the tool is subjected to a substantial load.